A firearm has a body formed of a unitary first shell and a unitary second shell. Each shell defines an open cavity and has a peripheral rim. The rims of the shells are connected together so that the cavities define a receiver chamber. A bolt reciprocates within the receiver chamber, and a barrel connected to the shells has a chamber positioned for operable engagement by the bolt. The shells may together define a gas tube, and may define a barrel receiving element, each shell having a barrel engagement element. Each shell may have a side panel with flat portions having a limited thickness, and elongated ribs of a greater thickness. The ribs may extend about the periphery of planar side panels, and may extend between the periphery and selected stress elements on the body including a buttstock mounting facility. A bolt handle slot may have opposed rows of spaced apart bolt handle guides in an alternating arrangement.